The properties of x-ray recordings are decisively determined by the spectrum or energy of the x-ray radiation, with which the object to be recorded is irradiated. This is because the capability of various materials in terms of scattering and absorbing x-ray radiation differs as a function of the spectrum or energy of the x-ray radiation.
It is therefore possible, by irradiating an object to be recorded with different spectra or energies of the x-ray radiation, to deduce the material composition of the object to be recorded. The irradiation of an object to be recorded with various spectra or energies of the x-ray radiation is used in particular in medical imaging and is also referred to as a “dual-energy” method. For instance, such a method enables bones or other tissue in the human body to be identified and increases the possibility of evaluating contrast agent-assisted images.
Modern medical devices such as for instance a computed tomography system realize a dual energy method with just one x-ray tube an as x-ray source. In this process use is generally made of so-called “kV-switching”, whereby the voltage of the x-ray tube is switched between two values up to several hundred times per second, wherein a voltage value specifies a specific spectrum and/or a specific energy of the x-ray radiation in each instance.
With a tomographic recording, the “kV switching” is embodied such that the voltage changes between the recording of two projections. With “kV switching”, in other words during a rotation of the x-ray tube, a plurality of projections at a first voltage and a plurality of projections at a second voltage, is recorded. Since the applied dose also changes with the voltage of the x-ray tube, the current of the x-ray tube and/or the exposure time per x-ray projection is in addition regulated. As a result, modern dual-energy methods are technically challenging.
DE 10 2004 051 820 A1 discloses a method for generating multi-energy images for a tomography device. The tomography device has a recording system for detecting projections of an object area, wherein the recording system includes an x-ray emitter for generating x-ray radiation by means of its voltage, current and exposure time of a predetermined x-ray dose. The voltage between a first voltage value and a second voltage value, which differs therefrom, is set alternately in order to detect high energy projections and low energy projections in each instance.
Furthermore, a further actuating value is set between a first control point assigned to the first voltage value and a second control point assigned to the second voltage value which differs therefrom. In the process the first control point is set as a function of the first voltage value and the second control point is set as a function of the second voltage value such that the respectively generated x-ray radiation has essentially the same x-ray dose.